In digital television systems, the television is provided with a set top box to receive and decode a broadcast digital data stream which contains program information for display on the television. The broadcast digital data stream may arise at the set top box via a satellite or cable system, via a digital terrestrial system, or via disk or tape. A disk or tape, such as a CD ROM in a personal computer, may provide digital video information for display on the monitor.
There are various known standards for digital video broadcasting (DVB) and one now commonly used standard is the MPEG-2 standard.
In the MPEG-2 DVB standard, data is encoded into transport packets. Each transport packet is defined by the standard as consisting of 188 bytes, comprising four header bytes and 184 payload bytes (“the data payload”). For transmission, the transport packets are time division multiplexed into a transport stream. At the receiver in the set top box, the transport stream is demultiplexed to recover the transport packets. Optionally, the transport packets may be scrambled and encoded with error correction information for transmission and then descrambled and error checked at the receiver. The data payload in the transport packets is, according to the MPEG-2 standard, one of two types. The first type is known is a packetised elementary stream (PES), and the second type is known as program specific information (PSI).
The packetised elementary streams (PESs) form the video, audio and private data information of the broadcast. A PES packet may contain all sorts of data, audio or video and also other information such as teletext or other user defined general data. The MPEG-2 transport stream is made up of one or more PESs (either video, audio or private). The MPEG-2 transport stream is primarily intended for the transport of TV programs over long distances. This type of stream can combine, in the same multiplex, many programs, each of them being composed of one or more PESs. In order that the receiver can cope with this mix of program information, the MPEG-2 standard defines all types of tables, which together make up the MPEG-2 program specific information (PSI).
At each decoder or set top box, the transport stream is decoded. To achieve the decoding of the transport stream, each set top box is provided with a transport interface, which provides an input interface between the transport stream input to the box and the actual MPEG-2 decoders which decode the audio and video information and sections broadcast. The transport interface demultiplexes the transport stream to retain only those transport packets, which are required by the particular set top box for decoding. The transport stream is a set of different services time division multiplexed and the purpose of the transport interface is to demultiplex them. At a front input end of the transport interface, a time demultiplex function is performed to separate the transport stream into its component transport packets.
Each transport packet has associated therewith in its header a packet identifier (PID) which identifies the type of packet and various information associated with the data in the packets including the type of packet (PES or PSI). Each particular receiver or set top box is only interested in receiving packets having packet identifiers of interest to the particular set top box, for instance those associated with the particular television program selected for viewing. Thus, once the incoming transport stream has been time demultiplexed to recover the transport packets, it is necessary to further demultiplex the transport packets to retain only those having packet identifiers required by the receiver.
The transport interface merely uses the header of PES transport packets to demultiplex them, and stores the data payload of the demultiplexed packets in the memory. The transport interface similarly demultiplexes PSI transport packets but then filters the sections of the demultiplexed packets to retain only sections required by the receiver, before storing the filtered sections in the memory without further processing.
It has been proposed that the MPEG data may be obtained from a memory instead of via a satellite or cable link. Previously a transport stream could be decoded and stored in memory. To replay that transport stream, it was necessary to use the CD FIFO of the MPEG decoder. This stored transport stream would be treated differently from the received satellite or cable transport stream. This means that the set top box has to have separate circuitry for dealing with the stored and received transport streams.